Interestingly enough, unsafe is the root reason Rust couldn't add Vale's Seamless Concurrency feature [0] which is basically a way to add a "parallel" loop that can access any existing data, without refactoring it or any existing code.

If Rust didn't have unsafe, it could have that feature. Instead, the Rust compiler assumes a lot of data is !Sync, such as anything that might indirectly contain a trait object (unless explicitly given + Sync) which might contain a Cell or RefCell, both of which are only possible because of unsafe.

Without those, without unsafe, shared borrow references would be true immutable borrow references, and Sync would go away, and we could have Seamless Concurrency in Rust.

I often wonder what else would emerge in an unsafe-less Rust!

Still, Given Rust's priorities (low level development) and the borrow checker's occasional need for workarounds, and the sheer usefulness of shared mutability, it was a wise decision for Rust to include unsafe.

[0] https://verdagon.dev/blog/seamless-fearless-structured-concu...